The research aims of this study are to relate the new areas of cytogenetics and human behavior genetics to problems in normal human development and psychopathology. We are investigating the nature of the relationship which might exist between the sex chromosome aneuploides in males (i.e., XXY, XYY) and subsequent personality development. Infants with these chromosome lesions-and infants serving as controls-were located through a chromosome screening of male neonates. These infants and children are being followed in a long-term developmental study with the aim of identifying reactive styles or developmental lags that might be related to the chromosome variations. The data, obtained through semistructured parental interviews (recorded verbatim) and direct observations of the child, are reduced to 16 operationally defined categories of behavior that represent the child's development and functioning in the most relevant aspects of psychosocial adaptation. Preliminary conclusions based on the systematic sampling method are tested independently by psychological testing, developmental assessments and formal experiments. Considerable controversy has surrounded the issue of the possible relationship which might exist between certain sex chromosome variations and personality development. This controversy has resulted from the fact that much of the evidence suggesting such a relationship has been obtained by surveying groups of individuals who demonstrated a particular personality deviation and finding an increased incidence of a specific chromosomal aneuploidy among them. The natural history of unselected individuals who carry such variations remains largely unknown. Should long-term longitudinal studies of affected individuals reveal any developmental risk associated with the variation, this knowledge can be directed to helping these individuals overcome developmental lags and realize their full developmental potential.